1. Field of the Invention
The invention relates to a power transmission system that includes an engine and a motor as driving force sources and also includes a transmission unit downstream of the driving force sources and, more particularly, to a technique for appropriately carrying out shift control over the transmission unit irrespective of an operating state of the driving force sources.
2. Description of the Related Art
There is known a power transmission system that includes an engine and a motor as driving force sources and that further includes a transmission unit arranged downstream of the driving force sources. The transmission unit establishes a plurality of gears having different speed ratios by frictional engagement devices. Japanese Patent Application Publication No. 2006-213149 (JP-A-2006-213149) describes a vehicle power transmission system, which is an example of the above power transmission system. The vehicle power transmission system includes (a) an engine, (b) an electric differential unit that includes a differential mechanism, having a differential input member and a differential output member, and a rotary machine, coupled to a rotating element of the differential mechanism so as to be able to transmit power, and that allows controlling a differential state between the rotational speed of the differential input member coupled to the engine and the rotational speed of the differential output member by controlling an operating state of the rotary machine, (c) a motor that is arranged so as to be able to transmit power to the differential output member of the electric differential unit, and (d) a transmission unit that is arranged between the differential output member and drive wheels. Then, in the above hybrid vehicle power transmission system, generally, a motor drive mode is established in an operating region in which engine efficiency is poor, such as a low vehicle speed region and a low output region, while an engine drive mode or an engine and motor drive mode is established in an operating region in which engine efficiency is good, such as intermediate and high vehicle speed regions and intermediate and high output regions. In the motor drive mode, the engine is stopped and only the motor is used to drive the vehicle. In the engine drive mode, the engine is started and only the engine is used to drive the vehicle. In the engine and motor drive mode, the engine and the motor are used to drive the vehicle. In addition, in the motor drive region in which only the motor is used to drive the vehicle, the engine is started as needed in order to charge a battery, carry out warm-up, or the like.
On the other hand, when the transmission unit shifts gears, the magnitude of torque transmission capacity (hydraulic pressure, or the like) and control pattern of the frictional engagement devices are determined in accordance with an input torque and a type of a shift (upshift, downshift, or the like). However, if start or stop of the engine overlaps a shift of the transmission unit, shift shock easily occurs. For example, when the motor drive mode is switched over to the engine drive mode or the engine and motor drive mode, the engine is driven for rotation (cranked) by the rotary machine, and reaction force is received by the motor. This suppresses fluctuations in driving force while making it possible to start the engine. When such start of the engine overlaps a shift of the transmission unit, there is a case where the timing of torque transmission capacity control of the frictional engagement devices with respect to the rising of input torque attended with start of the engine (start of operation of the engine) deviates and, as a result, shift shock occurs. Therefore, in JP-A-2006-213149, when start or stop of the engine overlaps a shift of the transmission unit, any one of the operations is carried out first, and the other operation is carried out next.
However, if any one of the operations is carried out first and the other operation is carried out next when start or stop of the engine overlaps a shift of the transmission unit, there is a problem that the response to a change in driving force required by the driver is poor.